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13: Infectious Diseases

Your Health Today, 7th Edition

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The Process of Infection

Infection: disease or condition caused by a microorganism

Microorganisms are the tiniest living organisms on earth that eat, reproduce, and die

Infection is considered an illness or disease if it interferes with your usual lifestyle or shortens your life

Process of infection often follows a typical course, with the length of each stage depending on the pathogen, an infectious agent capable of causing disease

Figure 13.1 Stages of infection.

Exposure: organism enters body

1. Incubation period (no symptoms)

2. Prodromal period (vague sense of not being well)

3. Invasive phase (full-blown illness)

4. Acme (peak of disease): either the immune system gains control, medical treatment occurs, or death ensues

5. Decline phase (symptoms brought under control)

6. Convalescent phase (body is repairing damage)

7. Sequelae (remaining symptoms the body can’t repair)

The Chain of Infection (1 of 2)

Chain of infection: the process by which an infectious agent, or pathogen, passes from one organism to another

Pathogens often live in large communities, called reservoirs

Some cannot survive in the environment and require a living host

To cause infection, they must have a portal of exit from the reservoir or host, and a portal of entry into a new host

If the transfer from the reservoir or host is carried out by an insect or animal, that organism is called a vector

The Chain of Infection (2 of 2)

Breaking the chain of infection at any point can either increase or decrease the risk of infection

Virulence: speed and intensity with which a pathogen is likely to cause an infection

Epidemic: widespread outbreak of a disease that affects many people

Figure 13.2 The Chain of infection.

Breaking the chain at any point decreases the risk of infection.

1. Susceptible host has factors that influence risk of infection, such as burns

2. Infectious agent or pathogen, such as a bacterium or virus

3. Reservoir, where the organism survives, such as in a human

4. Portal of exit, how the organism leaves, such as via blood

5. Mode of transmission, how it spreads, such as by direct contact

6. Portal of entry, how it enters a new host, such as through broken skin

Pathogens (1 of 3)

Viruses: tiny pathogens consisting of a genome (DNA or RNA) and protein covering

Examples: HIV; common cold (over 200 viruses); influenza; human papillomavirus (warts, cervical cancer); hepatitis A, B, C; polio; rabies

Bacteria: single-celled organisms that are spherical, rodlike, or spiral in shape

Staphylococcus aureus and MRSA; Neisseria meningitides (meningitis); chlamydia; gonorrhea; tuberculosis

Pathogens (2 of 3)

Prions: organisms believed to consist entirely of protein

Bovine spongiform encephalopathy (mad cow disease); Creutzfeldt-Jakob disease (CJD); Kuru

Fungi: single-celled or multicelled organisms

Candidiasis (yeast infection); tinea (athlete’s foot, ringworm); histoplasmosis

Helminths: parasitic worms that live on or in host

Hookworm; pinworm; tapeworm; liver flukes

Pathogens (3 of 3)

Protozoa: single-celled organisms that generally live independently of host

Giardia; toxoplasmosis; amebiasis; malaria; trichomoniasis

Ectoparasites: complex organisms that usually live on the host’s skin

Fleas; ticks; lice; scabies; bed bugs

External Barriers

Physical barriers

Skin is the body’s first line of defense

Nasal passages and ear canals are protected by hair

Lungs are protected by the cough reflex and cilia

Chemical barriers

Saliva contains proteins that break down bacteria

Stomach acids, difficult for most organisms to survive

Small intestines contain bile and enzymes that break down pathogens

Vagina is slightly acidic, discouraging the growth of abnormal bacteria

The Immune System

Immune system: complex set of cells, chemicals, and processes that protects the body against pathogens when they succeed in entering the body

Innate immune system: part of the immune system designed to rapidly dispose of pathogens in a nonspecific manner

Acquired immune system: highly specialized response that recognizes specific targets

Innate Immune System

Acute inflammatory response: the body’s initial reaction to tissue damage, bringing blood to the site of injury or infection

Cells of the innate immune system:

Neutrophils and macrophages: white blood cells that travel to areas of infection or tissue damage and digest damaged cells, foreign particles, and bacteria

Natural killer cells: white blood cells that recognize and destroy virus-infected cells or those that have become cancerous

Acquired Immune System (1 of 2)

Develops as you are exposed to potential infections and vaccinations

Lymphocytes: white blood cells that circulate in the bloodstream and lymphatic system

If lymphocytes encounter an antigen—a marker on the surface of a foreign substance—they rapidly duplicate and “turn on” their specific function

To main types of lymphocytes: T cells and B cells

Acquired Immune System (2 of 2)

T cells monitor events inside cells

Helper T cells “read” cells’ infection messages and trigger production of killer T cells and B cells

Killer T cells attack and kill foreign cells and infected body cells

Suppressor T cells slow down and halt the immune response when the threat has been handled

B cells monitor the blood and tissue fluids

When they encounter a specific antigen, they mature and produce antibodies: proteins that bind to specific antigens and trigger their destruction

Immunity

After surviving infection by a pathogen, you often acquire immunity to any future infections by the same pathogen

B and T cells become memory cells when exposed to an infectious agent, allowing recognition and quick action to destroy the invader

Vaccine: preparation of weakened or killed microorganisms administered to confer immunity

Protects you by stimulating an immune response

Protects society by shrinking the reservoir of infectious agents

Recommended Adult Immunizations (Figure 13.5) (1 of 2)

Recommended as of 2017 for all persons who meet the age requirement and lack documentation of vaccination or past infection (all covered by the Vaccine Injury Compensation Program):

Influenza: all adults, 1 dose annually; covered by the Vaccine Injury Compensation Program

Tetanus, diphtheria, pertussis (Td/Tdap): all adults, substitute Tdap for Td once, then Td booster every 10 years

Varicella: all adults, lifetime, 2 doses

Human papillomavirus (HPV), Female: aged 19 to 26, 3 doses

Human papillomavirus (HPV), Male: aged 19 to 21, 3 doses

Zoster: aged 60 and over, 1 dose

Measles, mumps, rubella (MMR): aged 19 to late 50s, 1 or 2 doses depending on indication

Pneumococcal 13-valent conjugate (PCV13): aged 65 and older, 1 dose

Pneumococcal polysaccharide (PPSV23): aged 65 and older, 1 dose

Source: Centers for Disease Control and Prevention (August 30, 2017). Recommended Adult Immunization Schedule for Adults Aged 19 Years and Older, by Vaccine and Age Group, United States, 2017. www.cdc.gov/vaccines/schedules/hcp/imz/adult.html.

Recommended Adult Immunizations (Figure 13.5) (2 of 2)

Recommended as of 2017 for persons with a risk factor (some covered by the Vaccine Injury Compensation Program):

Human papillomavirus (HPV), Male: aged 22 to 26 years, 3 doses

Pneumococcal 13-valent conjugate (PCV13): aged 19 to 65 years

Pneumococcal polysaccharide (PPSV23): aged 19 to 65 years, 1 or 2 doses depending on indication

Hepatitis A: any age, 2 or 3 doses depending on indication

Hepatitis B: any age, 3 doses

Meningococcal 4-valent conjugate (MenACWY) or polysaccharide (MPSV4): any age, 1 or more doses depending on indication

Meningococcal B (MenB): any age, 2 or 3 doses depending on vaccine

Haemophilus influenza type b (Hib): any age, 1 or 3 doses depending on indication

Risk Factors for Infection (1 of 2)

Controllable risk factors:

Eating a balanced diet

Exercising

Getting enough sleep

Managing stress properly

Receiving vaccinations, when available

Good hygiene

Protecting skin from damage

Avoiding tobacco and environmental tobacco smoke

Risk Factors for Infection (2 of 2)

Uncontrollable risk factors:

Age

There is higher risk at both ends of the lifespan

Infants have passive immunity—temporary immunity from antibodies in pregnancy and breastfeeding

Genetics

Uncontrollable sociocultural factors

Overcrowded living environments

Poverty

Disruption of Immunity

Immune system can malfunction

Autoimmune diseases: the immune system mistakenly identifies a part of the body as “nonself” and creates an immune response, causing damage to body cells and tissues

Allergies: the body identifies a harmless foreign substance as an antigen and creates an immune response

Anaphylactic shock: life-threatening systemic allergic response requiring immediate medical attention

Immunity and stress: long-term stress, especially, can suppress the immune system, leading to illness

Changing Patterns in Infectious Disease

Because of the dramatic declines in the death rate from infectious diseases during the 20th the U.S. surgeon general in 1969 declared that it was time to close the book on infectious diseases

He was wrong

Within a little more than 10 years, HIV was causing perplexity and alarm in hospitals in several U.S. cities

Today there are new infections, changes in patterns of old ones, and antibiotic resistance in still others

Food-Related Pathogen Transmission

More than 250 organisms are associated with food-related illnesses

Complexity of the U.S. food system means most foods travel a thousand miles or more before reaching the table

Risk of contaminated food is increased

Number of people that can be infected is increased

Difficulty of tracking infection back to the source also increases

Behavior-Related Pathogen Transmission

Travel: SARS outbreak (2003)

Climate change: changing patterns of infectious disease, especially water-borne and vector-borne diseases

Sexual behavior: three key factors in exposure to an STI are partner variables, personal susceptibility variables, and sex act variables

Illicit drug use: Use of contaminated needles and syringes; hepatitis C

Figure 13.6 Death rate from infectious diseases, United States, 20th century.

Access the text alternative for these images

Sources: Adapted from Achievements in Public Health, 1990–1999: Control of Infectious Disease. Morbidity and Mortality Weekly Report, 48 (29), p. 621, www.edc.gov/mmwr; Armstrong, G.L., Conn, L.A., and Pinner, R.W. (1999). Trends in Infectious Disease Mortality in the United States During the 20th Century. Journal of the American Medical Association, 281, pp. 61–66; American Water Works Association (1973). Water Chlorination Principles and Practices: AWWA Manual M20. Denver, CO: AWWA; Hansen, V., Oren, E., Dennis, L.K. et al. (2016). Infectious Disease Mortality in the United States, 1980–2015. Journal of the American Medical Association. 316(20):2149-2151.

Antibiotic Resistance

Antibiotic: drug that works by killing or preventing the growth of bacteria

Antibiotic resistance: lessened sensitivity to the effects of an antibiotic

Two factors are believed to account for antibiotic resistance:

Frequency with which resistant genes arise naturally among bacteria through mutation

Inappropriate use of antibiotics in health care, home care, and food production

Vaccination Controversies

As vaccine-prevented diseases become less common, people begin to question the necessity and safety of the vaccines

Serious reactions to currently recommended vaccinations are very rare

If rates of vaccination drop, the likelihood of a disease recurrence increases

Global Infectious Diseases

Four leading causes of global infectious disease mortality:

Pneumonia: infection of the lungs or lower respiratory tract; can be viral or bacterial

Leading cause of death in children after the first month; leading cause of death in low-income countries; and third most common cause of death for all ages worldwide

Diarrhea: kills an estimated 499,000 children per year

Tuberculosis: world’s most common infectious disease

Latent infection: not currently active but could reactivate

Malaria: mosquito-borne disease that caused 429,000 deaths in 2015

Infectious Diseases on Campus

Pertussis (whooping cough): infection of the respiratory tract that is highly contagious

Staphylococcus aureus skin infections: from a common bacterium carried on the skin or in the noses of healthy people

Urinary tract infections (UTIs): most common bacterial infection in wo`

Sexually Transmitted Infections

Sexually transmitted infections (STI) are spread predominantly through sexual contact

Preferred terminology over sexually transmitted disease (STD), because often there are no symptoms

Primary pathogens: viruses and bacteria

HIV/AIDS (1 of 3)

Cause: Human immunodeficiency virus (HIV) attacks cells of the immune system, especially macrophages and CD4 cells (a subcategory of helper T cells)

Uses the cell’s DNA to replicate itself

HIV/AIDS (2 of 3)

Course of the disease:

Early symptoms are easily mistaken for other infections

Eventually the immune system can no longer function fully, signaling the onset of acquired immune deficiency syndrome (AIDS)

Opportunistic infections produce symptoms

Rapid weight loss

Cough

Night sweats

Diarrhea

Rashes or skin blemishes

Memory loss

HIV/AIDS (3 of 3)

Methods of transmission:

Sexual conduct

Injection drug use

Contact with infected blood or body fluids

Mother-to-child transmission

Universal precautions are taken in health care settings

Figure 13.7 Adults and children estimated to be living with HIV in 2016.

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Source: Joint United Nations Programme on HIV and AIDS (UNAIDS). Core Epidemiology Slides. http://aidsdatahub.org/sites/default/files/publication/UNAIDS_Global_AIDS_Update_2017_Core_Epidemiology_Slides.pdf

HIV Testing

Many kinds of tests can now be done

CDC recommends testing at least once for anyone between the ages of 13 and 64

HIV testing is particularly recommended if you:

Are pregnant or planning to get pregnant

Have had sex with someone who is HIV positive or you did not know the person’s status

Have used IV drugs and shared drug equipment

Have exchanged sex for drugs or money

Have been diagnosed with another STI

Have been sexually assaulted

Management of HIV/AIDS

Antiretroviral agents do not cure the infection, but they slow the rate of replication and destruction, prolonging life and improving quality of life

Drug cocktails: complicated drug combinations that combat the development of resistant viral strains

Complexity, cost, and risk of side effects increase

Prevention

Reducing the risk of infection

Adult male circumcision

Vaccine trials are under way

Empowering women

New prevention strategies continue to be developed

Treatment as Prevention (TasP)

Post-exposure Prophylaxis (PEP)

Pre-exposure Prophylaxis (PrEP)

Bacterial STIs

Chlamydia: most common bacterial STI; young women at greatest risk

Gonorrhea: highest rates in young women; rates in Blacks 4.4 times higher than Whites

Pelvic inflammatory disease (PID): infection of uterus, fallopian tubes, and/or ovaries

Syphilis: if untreated, can lead to serious complications

Bacterial vaginosis (BV): alteration of the normal vaginal flora

Viral STIs (1 of 2)

Human papillomavirus (HPV):

Most common STI in the U.S; more than 40 types

HPV is transmitted by skin-to-skin contact, usually through penetrative vaginal or anal sex, but it can be transmitted with oral sex and from woman to woman

Most women with HPV are diagnosed through screening with the Papanicolaou smear (Pap test)

Viral STIs (2 of 2)

Genital herpes

Two strains of herpes simplex virus are HSV-1 and HSV-2

HSV-2 is associated with a higher likelihood of recurrent symptoms than HSV-1

No cure; prevention particularly important

Hepatitis: inflammation of the liver

Most common viral causes are hepatitis A, B, and C

A safe and effective vaccine is available for hepatitis A and B

Most hepatitis B infections in the U.S. are sexually transmitted

Other STIs

Trichomoniasis: caused by a protozoan; transmitted from person to person by sexual activity

Candidiasis: vaginal yeast infection

Pubic lice and scabies: in adults, most often sexually transmitted

Prevention of Infectious Diseases (1 of 3)

Support your immune system by adopting healthy lifestyle practices

Cover your cough

Avoid touching your face or mouth

Get an annual flu shot and booster vaccines as recommended

Minimize your use of antibiotics

If exposed, minimize chances of passing it on

Prevention of Infectious Diseases (2 of 3)

Practice the ABCDs of STI prevention:

A for abstain: Abstain from sex until you are ready for a long-term relationship, and abstain between relationships

B for be faithful: Be faithful and maintain a monogamous relationship

C for condoms: Use condoms

D for detection: Promote detection by being tested and following recommended screening guidelines

Prevention of Infectious Diseases (3 of 3)

When you travel, learn about potential infections at your destination

Take steps to prevent new diseases from taking hold in your community

In Review

What causes infection, and how does the body protect itself from infectious diseases?

How are infectious diseases changing?

What are the most common infectious diseases?

What are the most serious and most common sexually transmitted infections?

How can infectious diseases be prevented?

Accessibility Content: Text Alternatives For Images

Appendix A

Image Descriptions for Unsighted Students

Figure 13.6 Death Rate, Infectious Diseases Text Alternative

In 1900, deaths from all causes was over 1,600 per 100,000 population. Deaths from noninfectious causes was about 900 per 100,000. Deaths from infectious disease causes was about 800 per 100,000.

A sudden peak in infectious disease deaths (and in all deaths), at over 900 per 100,000, occurred as a result of the influenza pandemic of 1918; but by 1920 the rate of infectious disease death had fallen to below 400 per 100,000. Noninfectious causes climbed somewhat.

From the early 1950s to the year 2015, the rate of deaths from infectious disease in the U.S. remained below 100 per 100,000 population. Rates for all causes have hovered in the 800 to 1,000 per 100,000 range. Rates for noninfectious causes have remained just below the rates for all causes.

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Figure 13.7 Adults and Children with HIV Text Alternative

In North America and Western and Central Europe, an estimated 2.1 million people were living with HIV in 2016 (2.0 million to 2.3 million)

In the Caribbean, 310,000 (280,000 to 350,000)

In Latin America, 1.8 million (1.4 million to 2.1 million)

In Eastern Europe and Central Asia, 1.6 million (1.4 million to 1.7 million)

In Asia and the Pacific, 5.1 million (3.9 million to 7.2 million)

In the Middle East and North Africa, 230,000 (160,000 to 380,000)

In Western and Central Africa, 6.1 million (4.9 million to 7.6 million)

In Eastern and Southern Africa, 19.4 million (17.8 million to 21.1 million)

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